1. Field of the Invention
The present invention relates to an optical transmitter, and more particularly to an EML (Electro-absorption Modulator Laser) optical transmitter.
2. Description of the Related Art
As the transmission speed increases in an optical system, optical signals are influenced by a chirp. As a result, if a semiconductor laser used for loading optical signals is driven in a direct-modulation type transmitter, the transmission characteristics of optical signals tend to deteriorate. As such, an optical modulation is typically performed using an external modulator. However, the usage of an external modulator increases the cost of optical transmitters. An EML is a module consist of a semiconductor laser and a modulator integrated on a substrate, and is widely being used as an optical transmitter due to its economical implementation.
In general, as far as an optical transmission is concerned, excellent transmission characteristics are obtained if transmitted optical signals have a negative chirp and a large extinction ratio at the same time. However, in the case of implementing the EML, the extinction ratio and the chirp characteristics are varied greatly according to the range of a bias and the applied voltage. If the extinction ratio is increased, a positive chirp is increased. Then, the transmission characteristics deteriorate drastically according to the transmission distance. “Extinction ratio” represents a ratio (Imax/Imin) between the maximum intensity (Imax) and the minimum intensity (Imin) of transmitted light as the intensity of transmitted light is varied in an optical modulator. Hence, the extinction ratio is a measure of the characteristics of an optical modulator. Therefore, when using an EML for transmission, it is customary to decrease the positive chirp in spite of the decrease in the extinction ratio.
FIG. 1 shows the configuration of a transmission link employing a conventional EML optical transmitter. As shown, the transmission link comprises an electric signal generator 11 for generating electric signals; an EML 12 for generating light signals for transmission; optical fibers 13 for transferring optical signals from the EML 12; and, an optical receiver 14 for receiving the optical signals from the optical fibers 13.
FIG. 2 illustrates the characteristic curves of optical outputs and chirps graphically according to the applied voltages of a typical EML. In the drawing, the X-axis represents the applied voltages, the Y-axis on the right-hand side represents the intensity of chirp signals, and the Y-axis on the left-hand side represents a relative transmission, which indicates the intensity of modulated optical signals per the intensity of applied electric signals.
As shown in FIG. 2, the EML has a negative chirp when the applied voltage is over 0.7 V, and a positive chirp when the applied voltage is below 0.7 V. In order to obtain a large extinction ratio, the width of the applied voltage must be increased. This increases the swing width up to 0-0.7 V regions, which include the majority of positive chirps. Note that the regions of the increased positive chirp affects the transmission performances.
In general, although a large positive chirp is no problem before a transmission, as the transmission distance increases, the transmission characteristics deteriorate drastically. Therefore, for the purpose of improving the transmission performances, the positive chirp must be reduced. This means that the swing width of the applied voltage must be reduced and, to that end, the extinction ratio must be reduced.
Therefore, there is a need to improve the transmission characteristics of transmitters by reducing the positive chirp and the extinction ratio simultaneously.